As demands for VLSI devices having higher density and improved precision are further increasing in recent years, EUV lithography, which is an exposure technique using an extreme ultraviolet (hereinafter, referred to as “EUV”) light, is considered promising. EUV light indicates light in a wavelength range of a soft X-ray region or a vacuum ultraviolet region, specifically, light having a wavelength of about 0.2 to 100 nm.
A reflective mask used in such lithography has formed on, e.g., a glass or silicon substrate a multilayer reflective film for reflecting an exposure light, wherein the multilayer reflective film has formed therein a pattern of an absorber film for absorbing an exposure light. In an exposure machine for performing pattern transfer, light striking the reflective mask mounted on the exposure machine is absorbed by portions of the multilayer reflective film having the absorber film pattern and reflected by portions of the multilayer reflective film having no absorber film pattern. Then, the reflected light image is transferred through a reflection optical system onto a semiconductor substrate, such as a silicon wafer.
For achieving a semiconductor device having an increased density and improved precision using the above reflective mask, the reflective region in the reflective mask (the surface of the multilayer reflective film) is required to have a high reflectance with respect to an EUV light which is an exposure light.
The above-described multilayer reflective film is a multilayer film comprising elements having different refractive indices, which are periodically laminated, and, generally, a multilayer film is used in which a thin film of a heavy element or a compound thereof and a thin film of a light element or a compound thereof are alternately laminated in about 40 to 60 cycles of the layers. For example, as a multilayer reflective film for an EUV light having a wavelength of 13 to 14 nm, a Mo/Si periodically laminated film in which a Mo film and a Si film are alternately laminated in about 40 cycles of the layers is preferably used. Mo is easily oxidized in air which decreases the reflectance of the multilayer reflective film, and therefore the Si film constitutes the uppermost layer of the multilayer reflective film.
As a reflective mask used in the EUV lithography, for example, there is a reflective mask for exposure described in Patent Document 1 below. Specifically, Patent Document 1 has proposed a reflective photomask characterized by having: a substrate; a reflective layer, formed on the substrate, comprising a multilayer film in which two different films are alternately laminated; a buffer layer, formed on the reflective layer, comprising a ruthenium film; and an absorber pattern, formed on the buffer layer so as to have a predetermined pattern form, comprising a material capable of absorbing soft X-rays.
The above-described buffer layer is also called a protective film. When forming the absorber pattern, a part of the absorber film is etched through a resist, and, to ensure the formation of the absorber pattern, the absorber film is subjected to slight over etching, and therefore the film present under the absorber film is inevitably etched. In this instance, to prevent the multilayer reflective film under the absorber film from suffering a damage, a protective film is formed.
With respect to the protective film, further, from the viewpoint of suppressing the formation of a diffused layer (which leads to a reduction of the reflectance of the multilayer reflective film) between the Si layer constituting the surface layer of the multilayer reflective film and the protective film, a protective film comprising a Ru alloy having Zr or B added to Ru has been proposed (Patent Document 2).
Further, for solving a problem that, in the steps conducted when manufacturing a mask blank or a mirror or in the steps conducted when manufacturing a photomask from the mask blank (for example, cleaning, defect inspection, heating step, dry etching, and defect correction steps), or in the EUV exposure, the protective film and further the uppermost layer of the multilayer reflective film (Si layer in the case of a Mo/Si multilayer reflective film) are oxidized and reduce the EUV light reflectance, the formation of an intermediate layer containing Si and O in predetermined amounts between the Mo/Si multilayer reflective film and the Ru protective film has been proposed (Patent Document 3).
Patent Document 4 discloses a reflective mask blank having: a substrate; a multilayer reflective film, formed on the substrate, to reflect an exposure light; a protective film, formed on the multilayer reflective film, to protect the multilayer reflective film; and an absorber film, formed on the protective film, to absorb an exposure light, the reflective mask blank being characterized in that the protective film comprises ruthenium (Ru), or a ruthenium compound containing ruthenium (Ru) and at least one member selected from molybdenum (Mo), niobium (Nb), zirconium (Zr), yttrium (Y), boron (B), titanium (Ti), and lanthanum (La), wherein a thermal diffusion suppressing film comprising a material having a refractive index (n) of more than 0.90 and having a linear attenuation coefficient (k) of less than −0.020 is formed between the multilayer reflective film and the protective film.
The above documents further indicate that the multilayer reflective film may be subjected to heat treatment at 50 to 150° C., for example, to reduce the film stress of the multilayer reflective film, and that, in the working Examples, the multilayer reflective film was subjected to such heat treatment. Afterwards, the individual interfaces between the Si film as the uppermost layer of the multilayer reflective film, the thermal diffusion suppressing film, and the RuNb protective film were examined by means of a transmission electron microscope, and, as a result, a diffused layer was not confirmed in any of the interfaces.